Shift register circuit

ABSTRACT

An object is to enhance the driving capability and improve the operating speed of a unit shift register applicable to a scanning line driving circuit having a partial display function. A unit shift register forming a gate line driving circuit includes a first transistor that supplies a first clock signal to a first output terminal, a second transistor that supplies a second clock signal to a second output terminal, a third transistor that charges the gate of the first transistor in response to activation of a shift signal of the previous stage, and a fourth transistor connected between the gate of the first transistor and the gate of the second transistor. The first clock signal and the second clock signal have the same phase, and only the second clock signal is activated in a particular period (a display ineffective period).

CROSS REFERENCE TO RELATED APPLICATION

The present divisional application claims the benefit of priority under35 U.S.C. §120 to application Ser. No. 13/020,475, filed on Feb. 3,2011, now U.S. Pat. No. 8,379,790, which claimed the benefit of priorityunder 35 U.S.C. §119 from Japanese Patent Application No. 2010-107906,filed on May 10, 2010. The entire contents of application Ser. No.13/020,475 is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to shift register circuits applicable toscanning line driving circuits, and particularly to a shift registercircuit applicable to a scanning line driving circuit used in an imagedisplay apparatus having a partial display function and formed only offield-effect transistors of a same conductivity type.

2. Description of the Background Art

In an image display apparatus (hereinafter referred to as “a displayapparatus”) such as a liquid-crystal display apparatus, a gate line(scanning line) is provided for each pixel row (pixel line) on a displaypanel where a plurality of pixels are arranged in a matrix, and the gatelines are sequentially selected and driven in cycles of one horizontalperiod of a display signal to update the displayed image. For a gateline driving circuit (scanning line driving circuit) for sequentiallyselecting and driving the pixel lines or gate lines, a shift registerthat performs shift operation circulating in one frame period of thedisplay signal can be used.

In the field of display apparatuses, techniques are known which reducepower consumption by making “partial display” by limiting the display toonly a part of the screen. Specifically, the techniques disclosed inJapanese Patent Application Laid-Open Nos. 2008-140490 (hereinafterPatent Document 1) and 2008-58939 use shift registers formed only offield-effect transistors of the same conductivity type. When a shiftregister provided in a display apparatus is formed only of transistorsof the same conductivity type, the number of process steps in themanufacturing process of the display apparatus can be reduced to reducemanufacturing costs.

A shift register as a gate line driving circuit is formed of a pluralityof cascade-connected shift register circuits provided respectively forindividual pixel lines or gate lines. In this specification, for thesake of convenience of description, each of the plurality of shiftregister circuits forming such a gate line driving circuit is referredto as “a unit shift register”. That is to say, an output signal of eachunit shift register of a gate line driving circuit is supplied to a gateline, and also inputted to a unit shift register in a next or followingstage.

For example, Patent Document 1 discloses a scanning line driving circuitapplicable to partial display, and FIG. 3 thereof shows the circuitconfiguration of a unit shift register (Ua) forming it. The unit shiftregister (Ua) outputs a scanning signal (Y) for driving a scanning lineand a transfer signal (G) sent to the unit shift register in the nextstage. In the unit shift register (Ua), a transfer signal generator(110A) for generating the transfer signal (G) always operates, while ascanning signal generator (120A) for generating the scanning signal (Y)is controlled to operate only in selected periods of pixel linesrequired to make display in partial display (display effective periods).

The scanning signal generator (120A) includes a first transistor (121)connected between a first clock terminal (P) and a scanning signaloutput terminal (y). The transfer signal generator (110A) includes asecond transistor (111) connected between a second clock terminal (A)and a transfer signal output terminal (g). The gates of the first andsecond transistors (121, 111) are connected to each other, and theyswitch on and off with the same timing.

Now, a second clock signal (YCK1/YCK2) inputted to the second clockterminal (A) of each unit shift register (Ua) is activated always ingiven cycles, but a first clock signal (P1/P2) inputted to the firstclock terminal (P) is activated in given cycles only in displayeffective periods, and it is maintained at an inactive level in theremaining periods (display ineffective periods). Thus, the scanningsignal (Y) is activated only in display effective periods. As a result,only part of the pixel lines are activated, and partial display on thescreen is achieved.

In the unit shift register (Ua) of FIG. 3 of Patent Document 1, acapacitance element (113) is connected between the source (transfersignal output terminal (g)) and the gate of the second transistor (111).This capacitance element (113) steps up the gate of the secondtransistor (111) when the transfer signal (G) is activated, and thusserves to enhance the driving capability (the capability of passingcurrent) of the second transistor (111). When the gate of the secondtransistor (111) is sufficiently stepped up, the second transistor (111)operates in a non-saturation region, and the potential of the activelevel of the transfer signal (G) becomes higher.

As stated above, the gates of the first and second transistors (121,111) are connected to each other, so that the driving capability of thefirst transistor (121) is also enhanced when the gate of the secondtransistor (111) is stepped up. Here, a node (α) to which the gates ofthe first and second transistors (121, 111) connect is referred to as “agate node”.

The first and second transistors (121, 111) each have gate capacitance(gate-source capacitance, gate-drain capacitance and gate-channelcapacitance). In a display effective period, when the first and secondtransistors (121, 111) turn on, the transfer signal output terminal (g)and the scanning signal output terminal (y) are both charged in responseto activation of the first clock signal (P1/P2) and second clock signal(YCK1/YCK2), and the gate capacitances of the first and secondtransistors (121, 111) both function to step up the gate node (α)together with the capacitance element (113).

However, in a display ineffective period, the second clock signal(YCK1/YCK2) is activated but the first clock signal (P1/P2) is notactivated. Accordingly, when the first and second transistors (121, 111)turn on, only the transfer signal output terminal (g) is charged but thescanning signal output terminal (y) is not charged. In this case, thegate capacitance of the second transistor (111) functions to step up thegate node (α), but the gate capacitance of the first transistor (121)serves to hinder it. When the gate node (α) is insufficiently stepped upfor this reason, the driving capability of the second transistor (111)of the unit shift register (Ua) is lowered in the display ineffectiveperiod. This hinders high-speed operation of the unit shift register(Ua), and lowers the operational margin of the scanning line drivingcircuit.

The first transistor (121) is required to have high driving capabilityto drive a gate line having large load capacitance, and its gate widthis set wide. Accordingly, the gate capacitance of the first transistor(121) has a large value (the gate capacitance is proportional to theproduct of the gate width and gate length), and this problem is likelyto occur.

SUMMARY OF THE INVENTION

An object of the present invention is to enhance the driving capabilityand improve the operating speed of a unit shift register that isapplicable to a scanning line driving circuit having a partial displayfunction.

A shift register circuit according to a first aspect of the presentinvention includes: an input terminal, first and second outputterminals, and first and second clock terminals; and first to fourthtransistors below. The first transistor supplies the first outputterminal with a first clock signal inputted to the first clock terminal.The second transistor supplies the second output terminal with a secondclock signal inputted to the second clock terminal. The third transistorcharges a first node to which a control electrode of the firsttransistor connects or a second node to which a control electrode of thesecond transistor connects, in response to activation of an input signalinputted to the input terminal. The fourth transistor is connectedbetween the first node and the second node. The first clock signal andthe second clock signal have a same phase, and a setting can be madesuch that only the second clock signal is activated in a particularperiod.

A shift register circuit according to a second aspect of the presentinvention includes: an input terminal, first and second outputterminals, and first and second clock terminals; and first to fourthtransistors below. The first transistor supplies the first outputterminal with a first clock signal inputted to the first clock terminal.The second transistor supplies the second output terminal with a secondclock signal inputted to the second clock terminal. The third transistorcharges a first node to which a control electrode of the firsttransistor connects, in response to activation of an input signalinputted to the input terminal. The fourth transistor charges a secondnode to which a control electrode of the second transistor connects, inresponse to activation of the input signal. The first clock signal andthe second clock signal have a same phase, and a setting can be madesuch that only the second clock signal is activated in a particularperiod.

A shift register circuit according to a third aspect of the presentinvention includes: first and second input terminals, first and secondoutput terminals, and first and second clock terminals; first and secondvoltage signal terminals respectively supplied with first and secondvoltage signals that are complementary to each other; and first to fifthtransistors below. The first transistor supplies the first outputterminal with a first clock signal inputted to the first clock terminal.The second transistor supplies the second output terminal with a secondclock signal inputted to the second clock terminal. The third transistorsupplies the first voltage signal to a first node to which a controlelectrode of the first transistor connects or a second node to which acontrol electrode of the second transistor connects, in response toactivation of a first input signal inputted to the first input terminal.The fourth transistor supplies the second voltage signal to the firstnode or the second node in response to activation of a second inputsignal inputted to the second input terminal. The fifth transistor isconnected between the first node and the second node. The first clocksignal and the second clock signal have a same phase, and a setting canbe made such that only the second clock signal is activated in aparticular period.

A shift register circuit according to a fourth aspect of the presentinvention includes: first and second input terminals, first and secondoutput terminals, and first and second clock terminals; first and secondvoltage signal terminals respectively supplied with first and secondvoltage signals that are complementary to each other; and first to sixthtransistors below. The first transistor supplies the first outputterminal with a first clock signal inputted to the first clock terminal.The second transistor supplies the second output terminal with a secondclock signal inputted to the second clock terminal. The third transistorsupplies the first voltage signal to a first node to which a controlelectrode of the first transistor connects, in response to activation ofa first input signal inputted to the first input terminal. The fourthtransistor supplies the second voltage signal to the first node inresponse to activation of a second input signal inputted to the secondinput terminal. The fifth transistor supplies the first voltage signalto a second node to which a control electrode of the second transistorconnects, in response to activation of the first input signal. The sixthtransistor supplies the second voltage signal to the second node inresponse to activation of the second input signal. The first clocksignal and the second clock signal have a same phase, and a setting canbe made such that only the second clock signal is activated in aparticular period.

According to the present invention, when the second output signal (shiftsignal) is activated, the control electrode of the second transistorthat charges the second output terminal and the control electrode of thesecond transistor that charges the first output terminal of the firstoutput signal (gate line driving signal) are electrically separated.Accordingly, the driving capability of the second transistor can be kepthigh irrespective of whether or not it is a period in which the firstoutput signal is activated (a display effective period).

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram illustrating the configuration of aliquid-crystal display apparatus;

FIG. 2 is a diagram illustrating the configuration of a gate linedriving circuit according to a preferred embodiment of the presentinvention;

FIG. 3 is a circuit diagram illustrating a unit shift register of thepreferred embodiment;

FIG. 4 is a timing chart illustrating the operation of the gate linedriving circuit according to the preferred embodiment;

FIG. 5 is a circuit diagram of a unit shift register according to afirst modification of the preferred embodiment;

FIG. 6 is a circuit diagram of a unit shift register according to asecond modification of the preferred embodiment;

FIG. 7 is a circuit diagram of a unit shift register according to athird modification of the preferred embodiment;

FIG. 8 is a circuit diagram of a unit shift register according to afourth modification of the preferred embodiment;

FIG. 9 is a circuit diagram of a unit shift register according to afifth modification of the preferred embodiment; and

FIG. 10 is a circuit diagram of a unit shift register according to asixth modification of the preferred embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, the preferred embodiments of the present invention will bedescribed referring to the drawings. In order to avoid redundantrepetitions of description, elements having the same or correspondingfunctions are shown with the same reference characters in the drawings.

The transistors used in the preferred embodiments are insulated-gatefield-effect transistors. In an insulated-gate field-effect transistor,the electric conductivity between the drain region and source region inthe semiconductor layer is controlled by the electric field in the gateinsulating film. The material of the semiconductor layer in which thedrain region and source region are formed can be polysilicon, amorphoussilicon, organic semiconductor like pentacene, single crystal silicon,or oxide semiconductor like IGZO (In—Ga—Zn—O), for example.

As is well known, a transistor is an element that has at least threeelectrodes including a control electrode (a gate (electrode) in a narrowsense), one current electrode (a drain (electrode) or a source(electrode) in a narrow sense), and the other current electrode (asource (electrode) or a drain (electrode) in a narrow sense). Atransistor functions as a switching element in which a channel is formedbetween the drain and source when a given voltage is applied to thegate. The drain and the source of a transistor are structured basicallythe same, and their names are changed according to the applied voltagecondition. For example, with an N-type transistor, an electrode of arelatively higher potential (hereinafter also referred to as “level”) isreferred to as a drain, and an electrode with a lower potential isreferred to as a source (a P-type transistor has the opposite relation).

Unless specifically noted, such transistors may be ones formed on asemiconductor substrate, or may be thin-film transistors (TFTs) formedon an insulating substrate such as glass. Substrates on whichtransistors are formed may also be single crystal substrates, orinsulating substrates such as SOI,glass, resin, etc.

The gate line driving circuit of the present invention is formed byusing transistors of a single conductivity type. For example, an N-typetransistor goes into an active state (on state, conducting state) whenthe gate-source voltage goes to H (High) level higher than the thresholdvoltage of that transistor, and goes into an inactive state (off state,non-conducting state) at L (Low) level lower than the threshold voltage.Accordingly, in a circuit using N-type transistors, H level of a signalis an “active level”, and L level is an “inactive level”. Also, a nodein a circuit using N-type transistors is charged to H level and a changefrom an inactive level to active level occurs, and it is discharged to Llevel and a change from an active level to inactive level occurs.

On the other hand, a P-type transistor goes into an active state (onstate, conducting state) when the gate-source voltage goes to L levellower than the threshold voltage of the transistor (a negative valuebased on the source), and goes into an inactive state (off state,non-conducting state) at H level higher than the threshold voltage.Accordingly, in a circuit using P-type transistors, L level of a signalis an “active level”, and H level is an “inactive level”. Also, thecharging and discharging relations of nodes in a circuit using P-typetransistors are opposite to those of N-type transistors, and they arecharged to L level and a change from an inactive level to active leveloccurs, and discharged to H level and a change from an active level toinactive level occurs.

In this specification, a change from an inactive level to an activelevel is defined as “pull up”, and a change from an active level to aninactive level is defined as “pull down”. That is, in a circuit usingN-type transistors, a change from L level to H level is defined as “pullup”, and a change from H level to L level is defined as “pull down”, andin a circuit using P-type transistors, a change from H level to L levelis defined as “pull up”, and a change from L level to H level is definedas “pull down”.

Also, in this specification, “connect” between two elements, two nodes,or one element and one node includes connections that are made throughanother component (an element, switch, etc.) but that are substantiallyequivalent to direct connection. For example, even when two elements areconnected through a switch, the two elements are represented as“connected” when they can function in the same way as when they aredirectly connected.

In the present invention, clock signals having different phases(multi-phase clock signals) are used. In the description below, for thesake of simplicity, a certain interval is set between an active periodof one clock signal and the active period of the next activated clocksignal (Δt in FIG. 4). However, in the present invention, what isessential is that the active periods of clock signals do notsubstantially overlap, and the above-mentioned intervals may be omitted.For example, when the active level is H level, the timing of a fall ofone clock signal (a change from H level to L level) and the timing of arise of the next activated clock signal (a change from L level to Hlevel) may be the same timing.

Preferred Embodiment

FIG. 1 is a schematic block diagram illustrating the configuration of adisplay apparatus according to the present invention, and it shows theoverall configuration of a liquid-crystal display apparatus as a typicalexample of a display apparatus. The present invention is not limited toapplications to liquid-crystal display apparatuses, but can be widelyapplied to electro-optic apparatuses, such as electroluminescence (EL),organic EL, plasma displays, electronic paper, etc., as displayapparatuses that convert electric signals to light luminance.

The liquid-crystal display apparatus 100 includes a liquid-crystal arrayportion 10, a gate line driving circuit (scanning line driving circuit)30, and a source driver 40. As will become clear from the descriptionbelow, the shift register of this preferred embodiment is provided inthe gate line driving circuit 30.

The liquid-crystal array portion 10 includes a plurality of pixels 15arranged in a matrix. Gate lines GL₁, GL₂, . . . (collectively referredto as “gate lines GL”) are provided respectively for the rows of pixels(hereinafter also referred to as “pixel lines”), and data lines DL₁,DL₂, . . . (collectively referred to as “data lines DL”) are providedrespectively for the columns of pixels (hereinafter also referred to as“pixel columns”). As examples thereof, FIG. 1 shows pixels 15 on thefirst column and the second column on the first row, and thecorresponding gate line GL₁ and data lines DL₁ and DL₂.

Each pixel 15 includes a pixel switch element 16 provided between thecorresponding data line DL and a pixel node Np, and a capacitor 17 and aliquid-crystal display element 18 connected in parallel between thepixel node Np and common electrode node Nc. The alignment of the liquidcrystal in the liquid-crystal display element 18 changes according tothe voltage difference between the pixel node Np and common electrodenode Nc, and the display luminance of the liquid-crystal display element18 changes in response. Thus, it is possible to control the luminance ofeach pixel according to the display voltage transmitted to the pixelnode Np through the data line DL and pixel switch element 16. That is,intermediate luminance can be obtained as an intermediate voltagedifference between the voltage difference corresponding to the maximumluminance and the voltage difference corresponding to the minimumluminance is applied between the pixel node Np and the common electrodenode Nc. Thus, levels of luminance can be obtained by setting thedisplay voltage at levels.

On the basis of a given scanning cycle, the gate line driving circuit 30sequentially selects and activates the gate lines GL. The gateelectrodes of the pixel switch elements 16 are connected respectively tothe corresponding gate lines GL. While a particular gate line GL isbeing selected, the pixel switch elements 16 of the pixels connectedthereto are conductive and the pixel nodes Np are connected to thecorresponding data lines DL. Then, the display voltage transmitted tothe pixel node Np is held in the capacitor 17. Generally, the pixelswitch elements 16 are formed of TFTs formed on the same insulatingsubstrate (glass substrate, resin substrate, etc.) with theliquid-crystal display elements 18.

To the data lines DL, the source driver 40 outputs display voltage thatis set at levels by the display signal SIG as an N-bit digital signal.Herein, as an example, the display signal SIG is a 6-bit signal andformed of display signal bits DB0 to DB5. On the basis of the 6-bitdisplay signal SIG, for each pixel, 2⁶=64 levels of gray scale displayare possible. Furthermore, when one color display unit is formed ofthree pixels of R (Red), G (Green) and B (Blue), color display of about260,000 colors is possible.

As shown in FIG. 1, the source driver 40 includes a shift register 50,data latch circuits 52, 54, a gray scale voltage generating circuit 60,a decode circuit 70, and an analog amp 80.

In the display signal SIG, the display signal bits DB0 to DB5corresponding to the display luminance of each pixel 15 are seriallygenerated. That is, the display signal bits DB0 to DB5 at each timingindicate the display luminance in one pixel 15 in the liquid-crystalarray portion 10.

The shift register 50 instructs the data latch circuit 52 to capture thedisplay signal bits DB0 to DB5 with a timing synchronized with the cycleof switching of the setting of the display signal SIG. The data latchcircuit 52 sequentially captures the serially generated display signalSIG and holds a display signal SIG for one pixel line.

The latch signal LT inputted to the data latch circuit 54 is activatedwith the timing by which the display signal SIG for one pixel line iscaptured into the data latch circuit 52. In response, the data latchcircuit 54 captures the display signal SIG for one pixel line that isheld in the data latch circuit 52 at that time.

The gray scale voltage generating circuit 60 is formed of 63voltage-dividing resistors connected in series between high voltage VDHand low voltage VDL and generates 64 levels of gray scale voltages V1 toV64.

The decode circuit 70 decodes the display signal SIG held in the datalatch circuit 54. On the basis of the results of decoding, the decodecircuit 70 selects voltages from among the gray scale voltages V1 toV64, and outputs them to the decode output nodes Nd₁, Nd₂, . . .(collectively referred to as “decode output nodes Nd”).

As a result, display voltages (one of the gray scale voltages V1 to V64)corresponding to the display signal SIG for one pixel line held in thedata latch circuit 54 are simultaneously (in parallel) outputted to thedecode output nodes Nd. FIG. 1 shows the decode output nodes Nd₁ and Nd₂corresponding to the first and second data lines DL₁ and DL₂ asexamples.

The analog amp 80 amplifies the analog voltages corresponding to thedisplay voltages outputted from the decode circuit 70 to the decodeoutput nodes Nd₁, Nd₂, . . . and outputs them to the data lines DL₁,DL₂, . . .

On the basis of a given scanning cycle, the source driver 40 repeatedlyoutputs display voltages corresponding to the series of display signalsSIG to the data lines DL, one pixel line at a time, and the gate linedriving circuit 30 sequentially drives the gate lines GL₁, GL₂, . . . insynchronization with the scanning cycle, whereby the liquid-crystalarray portion 10 displays images based on the display signal SIG.

FIG. 1 shows the configuration of a liquid-crystal display apparatus 100in which the gate line driving circuit 30 and the source driver 40 areintegrated with the liquid-crystal array portion 10. However, the gateline driving circuit 30 and the liquid-crystal array portion 10 may beintegrated together and the source driver 40 may be provided as anexternal circuit of the liquid-crystal array portion 10, or, the gateline driving circuit 30 and the source driver 40 may be provided asexternal circuitry of the liquid-crystal array portion 10.

FIG. 2 is a diagram illustrating the configuration of the gate linedriving circuit 30. This gate line driving circuit 30 is formed of ashift register composed of a plurality of cascade-connected unit shiftregisters SR₁, SR₂, SR₃, SR₄, . . . , SR_(n) (for the sake ofconvenience of description, the cascade-connected unit shift registersSR₁, SR₂, . . . are collectively referred to as “unit shift registersSR”). Each unit shift register SR is provided for one pixel line, or onegate line GL.

Each unit shift register SR has an input terminal IN, a first outputterminal OUT, a second output terminal OUTS, a reset terminal RST, afirst clock terminal CK, and a second clock terminal CKS. The firstoutput terminal OUT is connected to the corresponding gate line GL.

The first output terminal OUT of each unit shift register SR outputs agate line driving signal G (a first output signal) for driving thecorresponding gate line GL. The gate line driving signal G functions asa so-called vertical (or horizontal) scanning pulse.

The second output terminal OUTS of each unit shift register SR outputs ashift signal SH (a second output signal) defining the shift operation ofsignals of the gate line driving circuit 30. The input terminal IN ofeach unit shift register SR is supplied with, as an input signal, theshift signal SH outputted from the unit shift register SR in theprevious stage. However, the input terminal IN of the unit shiftregister SR₁ in the first stage is supplied with, as an input signal, astart pulse SP corresponding to the beginning of each frame period ofthe image signal.

The shift signal SH of each unit shift register SR is supplied also tothe reset terminal RST of the unit shift register SR in the previousstage. That is to say, the reset terminal RST of each unit shiftregister SR is supplied, as a reset signal, with the shift signaloutputted from the unit shift register SR in the next stage. However,the reset terminal RST of the unit shift register SR_(n) in the finalstage (the nth stage) is supplied, as a reset signal, with theabove-mentioned start pulse SP.

The first clock terminal CK of each unit shift register SR is suppliedwith a clock signal CLK or /CLK generated by a first clock signalgenerator 31A. In the example of FIG. 2, the clock signal CLK issupplied to the first clock terminals CK of the odd-numbered unit shiftregisters SR₁, SR₃, SR₅, . . . , and the clock signal /CLK is suppliedto the first clock terminals CK of the even-numbered unit shiftregisters SR₂, SR₄, SR₆, . . .

The second clock terminal CKS of each unit shift register SR is suppliedwith a clock signal CLKS or /CLKS generated by a second clock signalgenerator 31B. In the example of FIG. 2, the clock signal CLKS issupplied to the second clock terminals CKS of the odd-numbered unitshift registers SR₁, SR₃, SR₅, . . . , and the clock signal /CLKS issupplied to the second clock terminals CKS of the even-numbered unitshift registers SR₂, SR₄, SR₆, . . .

Hereinafter, for the sake of convenience of description, the clocksignals CLK, /CLK generated by the first clock signal generator 31 A arereferred to as “first clock signals”, and the second clock signals CLKS,/CLKS generated by the second clock signal generator 31 B are referredto as “second clock signals”.

The first clock signals CLK and /CLK are two-phase clock signals havingdifferent phases (the active periods do not overlap). The first clocksignals CLK and /CLK have opposite phases, and are controlled to bealternately activated with a timing synchronized with the scanningcycles of the display apparatus.

The second clock signals CLKS, /CLKS are also two-phase clock signalshaving different phases (the active periods do not overlap), and, likethe first clock signals CLK and /CLK, they are also controlled to bealternately activated with a timing synchronized with the scanningcycles of the display apparatus. The first clock signal CLK and thesecond clock signal CLKS have the same phase, and the first clocksignal/CLK and the second clock signal /CLKS have the same phase.

In each unit shift register SR of this preferred embodiment, the firstclock signals CLK, /CLK are used as signals for activating the gate linedriving signal G, and the second clock signals CLKS, /CLKS are used assignals for activating the shift signal SH.

The second clock signals CLKS, /CLKS are always activated in givencycles (corresponding to 2 horizontal scanning periods) in each frame,but the first clock signals CLK, /CLK are activated only in selectedperiods of pixel lines requiring display (display effective periods).That is to say, when a partial display is made, the first clock signalsCLK, /CLK are activated only in a display effective period, and they arekept at an inactive level in the remaining periods (display ineffectiveperiods). Accordingly, the first clock signals CLK, /CLK are signalshaving waveforms corresponding to part of the waveforms of the secondclock signals CLKS, /CLKS. When a normal display is made in the entirescreen, the first clock signals CLK, /CLK are activated always withgiven cycles like the second clock signals CLKS, /CLKS.

As will be fully described later, in synchronization with the secondclock signals CLKS, /CLKS, each unit shift register SR of the gate linedriving circuit 30 operates to transfer the signal inputted to the inputterminal IN (the start pulse SP or the shift signal SH of the previousstage) to the unit shift register SR in the next stage, while shiftingthe signals in time. As a result, the shift signals SH of the individualunit shift registers SR are sequentially activated, as SH₁, SH₂, SH₃, .. .

Also, when activating the shift signal SH, each unit shift register SRalso activates the gate line driving signal G in response to activationof the first clock signals CLK, /CLK. Since the first clock signal CLKand the second clock signal CLKS have the same phase and the first clocksignal /CLK and the second clock signal /CLKS also have the same phase,the gate line driving signals G₁, G₂, G₃, . . . are activatedrespectively with the same timing as the shift signals SH₁, SH₂, SH₃, .. . As a result, the gate lines GL₁, GL₂, GL₃, . . . are sequentiallyactivated. However, the first clock signals CLK, /CLK are activated onlyin display effective periods. Accordingly, when a partial display ismade, only the gate lines GL corresponding to the display effectiveperiod are activated.

FIG. 3 is a circuit diagram illustrating the configuration of a unitshift register SR of the present invention. In the gate line drivingcircuit 30, the plurality of cascade-connected unit shift registers SRare all configured substantially the same, so that the configuration ofa kth unit shift register SR_(k) will be illustrated as an example. Inthis preferred embodiment, the transistors forming the unit shiftregisters SR are all field-effect transistors of the same conductivitytype, and an example using N-type TFTs will be described herein.

As shown in FIG. 3, the unit shift register SR_(k) has the inputterminal IN, the first output terminal OUT, the second output terminalOUTS, the first clock terminal CK, the second clock terminal CKS, andthe reset terminal RST shown in FIG. 2, and it further has a firstpower-supply terminal Si supplied with a low-side power-supply potentialVSS, and second and third power-supply terminals S2 and S3 respectivelysupplied with high-side power-supply potentials VDD1 and VDD2. Thehigh-side power-supply potentials VDD1 and VDD2 may be at the samelevel.

In the description below, the low-side power-supply potential VSS is thereference potential of the circuit (VSS=0). However, in practical use, areference potential is set on the basis of the voltage of data writtento the pixels, and the high-side power-supply potentials VDD1 and VDD2are set at 17 V, for example, and the low-side power-supply potentialVSS is set at −12 V, for example.

In the unit shift register SR_(k), the output stage of the gate linedriving signal G_(k) includes a transistor Q1 that brings the gate linedriving signal G_(k) to an active level (H level) when it is a selectedperiod of the gate line GL_(k) and also a display effective period, anda transistor Q2 for keeping the gate line driving signal G_(k) at aninactive level (L level) in the non-selected periods of the gate lineGL_(k).

The transistor Q1 is connected between the first output terminal OUT andthe first clock terminal CK, and it activates the gate line drivingsignal G_(k) by supplying the first output terminal OUT with the firstclock signal (CLK or /CLK) inputted to the first clock terminal CK. Thetransistor Q2 is connected between the first output terminal OUT and thefirst power-supply terminal S1, and it keeps the gate line drivingsignal G_(k) at an inactive level by discharging the first outputterminal OUT to the potential VSS. The node to which the gate (controlelectrode) of the transistor Q1 connects is defined as “a node N1”, andthe node to which the gate of the transistor Q2 connects is defined as“a node N2”.

A capacitance element C (step-up capacitance) is provided between thegate and source of the transistor Q1 (i.e. between the first outputterminal OUT and the node N1). The capacitance element Ccapacitive-couples the first output terminal OUT and the node N1, inorder to enhance the step-up effect of the node N1 as the level of thefirst output terminal OUT rises. However, when the gate-channelcapacitance of the transistor Q1 is sufficiently large, the capacitanceelement C can be replaced by it, and can be omitted in this case.

The output stage of the shift signal SH_(k) includes a transistor Q1Sthat brings the shift signal SH_(k) to an active level (H level) in theselected period of the gate line GL_(k), and a transistor Q2S forkeeping the shift signal SH_(k) at an inactive level (L level) in thenon-selected periods of the gate line GL_(k).

The transistor Q1S is connected between the second output terminal OUTSand the second clock terminal CKS, and it activates the shift signalSH_(k) by supplying the second output terminal OUTS with the secondclock signal (CLKS or /CLKS) inputted to the second clock terminal CKS.The transistor Q2S is connected between the second output terminal OUTSand the first power-supply terminal S1, and it keeps the shift signalSH_(k) at an inactive level by discharging the second output terminalOUTS to the potential VSS. The gate of the transistor Q2S is connectedto the node N2. The node to which the gate of the transistor Q1Sconnects is defined as “a node N1S”.

In the unit shift register SR_(k) of this preferred embodiment, atransistor Q8 having its gate connected to the second power-supplyterminal S2 is connected between the node N1 and the node N1S.

A capacitance element CS (step-up capacitance) is provided between thegate and source of the transistor Q1S (i.e. between the second outputterminal OUTS and the node N1S). The capacitance element CScapacitive-couples the second output terminal OUTS and the node N1S, inorder to enhance the step-up effect of the node N1S as the level of thesecond output terminal OUTS rises. However, when the gate-channelcapacitance of the transistor Q1S is sufficiently large, the capacitanceelement CS can be replaced by it, and can be omitted in this case.

A transistor Q3 is connected between the node N1 and the secondpower-supply terminal S2, and its gate is connected to the inputterminal IN. The transistor Q3 functions to charge the node N1 inresponse to activation of the signal supplied to the input terminal IN(input signal). The drain of the transistor Q3 may be connected to thethird power-supply terminal S3.

A transistor Q4, having its gate connected to the reset terminal RST, isconnected between the node N1 and the first power-supply terminal S1.The transistor Q4 functions to discharge the node N1 in response toactivation of the signal supplied to the reset terminal RST (resetsignal). Also, a transistor Q5, having its gate connected to the nodeN2, is connected between the node N1 and the first power-supply terminalS1. The transistor Q5 functions to keep the node N1 at an inactive level(L level) by discharging the node N1 while the node N2 is at an activelevel (H level).

The circuit formed of the transistors Q3, Q4 and Q5 forms “a pull-updriving circuit” that drives the transistors Q1 and Q1S (output pull-uptransistors) by charging/discharging the node N1.

Also, a transistor Q6, having its gate connected to the thirdpower-supply terminal S3, is connected between the node N2 and the thirdpower-supply terminal S3 (i.e. the transistor Q6 is diode-connected). Atransistor Q7, having its gate connected to the node Ni, is connectedbetween the node N2 and the first power-supply terminal S1.

The on-state resistance of the transistor Q7 is set sufficiently smallerthan that of the transistor Q6 (i.e. its driving capability is setlarger). Accordingly, when the gate of the transistor Q7 (the node N1)goes to H level and the transistor Q7 turns on, the node N2 isdischarged to L level. On the other hand, when the node N1 goes to Llevel and the transistor Q7 turns off, the node N2 is charged by thetransistor Q6 to H level.

That is to say, the transistors Q6 and Q7 form a ratio-type inverterhaving the node N1 as an input end and the node N2 as an output end. Inthis inverter, the transistor Q6 functions as a load element and thetransistor Q7 functions as a driving element. This inverter forms “apull-down driving circuit” that drives the transistors Q2 and Q2S(output pull-down transistors) by charging/discharging the node N2.

The operation of the unit shift register SR_(k) of FIG. 3 will bedescribed. For the sake of simplicity of description, in the unit shiftregister SR_(k), it is assumed that the first clock signal CLK isinputted to the first clock terminal CK and the second clock signal CLKSis inputted to the second clock terminal CKS (for example, in FIG. 2,the unit shift registers SR₁, SR₃, etc correspond to this).

Also, the low-side power-supply potential VSS supplied to the firstpower-supply terminal S1 is assumed to be the reference potential (0[V]). Also, it is assumed that the L level potentials of the first clocksignals CLK, /CLK, second clock signals CLKS, /CLKS and the start pulseSP are all VSS (0 [V]). Also, the high-side power-supply potential VDD1supplied to the second power-supply terminal S2 and the high-sidepower-supply potential VDD2 supplied to the third power-supply terminalS3 are assumed to be equal to each other, and its value is taken as VDD.

Also, the H level potentials of the first clock signals CLK, /CLK,second clock signals CLKS, /CLKS and the start pulse SP are assumed tobe all equal, and its value is also taken as VDD. Also, the thresholdvoltages of the transistors are assumed to be all equal, and its valueis taken as Vth.

The operation of the unit shift register SR_(k) in a display effectiveperiod will be described here. In a display effective period, the firstclock signals CLK and /CLK are activated like the second clock signalsCLKS and /CLKS.

As an initial state of the unit shift register SR_(k), a state where thenode N1 is at L level (VSS) and the node N2 is at H level (VDD-Vth) isassumed (hereinafter this state is referred to as “a reset state”). Atthis time, the transistor Q1 is off and the transistor Q2 is on, and thefirst output terminal OUT (gate line driving signal G_(k)) is at L level(VSS). Also, the transistor Q8 is on, and the node N1S is at L level(VSS) as well as the node N1. Accordingly, the transistor Q1S is off andthe transistor Q2S is on, and the second output terminal OUTS (shiftsignal SH_(k)) is also at L level (VSS).

From this state, when the shift signal SH_(k−1) of the previous stagegoes to H level (VDD), the transistor Q3 in the unit shift registerSR_(k) turns on. At this time, the node N2 is at H level and thetransistor Q5 is also on, but since the on-state resistance of thetransistor Q3 is set sufficiently smaller than that of the transistor Q5(the driving capability is set sufficiently larger), and the potentialof the node N1 rises.

Then, the transistor Q7 starts conducting and the level of the node N2decreases. Then, the resistance value of the transistor Q5 rises and thelevel of the node N1 rapidly rises, and the transistor Q7 sufficientlyturns on. As a result, the node N2 goes to L level (approximately equalto VSS). In response, the transistor Q5 turns off, and the node N1 goesto H level (VDD-Vth). When thus the node N1 is at H level and the nodeN2 is at L level (hereinafter this state is referred to as “a setstate”), the transistor Q1 is on and the transistor Q2 is off.

Since the node N2 went to L level, the transistor Q2S turns off. Also,since the node N1 went to H level, the node N1S is charged through thetransistor Q8 in an on state and goes to H level (VDD-Vth). Accordingly,the transistor Q1S turns on.

When the shift signal SH_(k−1) of the previous stage returns to L level,the transistor Q3 turns off, and the transistors Q4 and Q5 are also off,and the nodes N1 and N1S are kept at H level with high impedance state(floating state).

Since it is in a display effective period, the first clock signal CLK isnext activated. When the first clock signal CLK goes to H level (VDD),the first output terminal OUT is charged through the transistor Q1 in anon state, and the level of the gate line driving signal G_(k) rises. Atthis time, the potential of the node N1 is stepped up due to thecoupling through the capacitance element C and the gate capacitance ofthe transistor Q1 (gate-drain capacitance, gate-source capacitance andgate-channel capacitance). Accordingly, even when the level of the firstoutput terminal OUT rises, the gate-source voltage of the transistor Q1is kept larger than the threshold voltage (Vth), and the transistor Q1is kept at low impedance.

Accordingly, the gate line driving signal G_(k) quickly goes to H levelfollowing the rise of the first clock signal CLK. Also, since thetransistor Q1 operates in a non-saturation region, the level of the gateline driving signal G_(k) rises to the same potential VDD as the clocksignal CLK. As a result, the gate line GL_(k) enters a selected state.

At the same time as the first clock signal CLK goes to H level, thesecond clock signal CLKS also goes to H level (VDD). Then, the secondoutput terminal OUTS is charged through the transistor Q1S in an onstate, and the level of the shift signal SH_(k) rises. At this time, thepotential of the node N1S is stepped up due to the coupling through thecapacitance element CS and the gate capacitance of the transistor Q1S(gate-drain capacitance, gate-source capacitance, and gate-channelcapacitance).

Accordingly, even when the level of the second output terminal OUTSrises, the gate-source voltage of the transistor Q1S is kept larger thanthe threshold voltage (Vth), and the transistor Q1S is kept at lowimpedance.

Accordingly, the shift signal SH_(k) quickly goes to H level followingthe rise of the second clock signal CLKS. Also, at this time, thetransistor Q1S operates in a non-saturation region and charges thesecond output terminal OUTS, so that the level of the shift signalSH_(k) rises to the same potential VDD as the second clock signal CLKS.

Since the step-up of the node N1 and the step-up of the node N1S takeplace at the same time, the gate of the transistor Q8 becomes higherthan the two current electrodes (source, drain). Accordingly, when thenodes N1 and N1S are stepped up, the transistor Q8 turns off, and thenodes N1 and N1S are electrically separated. Since the node N1S isconnected to a small number of transistors, the parasitic capacitancebecomes very small when separated from the node N1. Accordingly, thenode N1S is sufficiently stepped up and the rising rate of the shiftsignal SH_(k) is sufficiently high.

After that, the first clock signal CLK and the second clock signal CLKgoes to L level (VSS) together. Then, the first and second outputterminals OUT and OUTS are discharged through the transistors Q1 and Q1Sin an on state, and the gate line driving signal G_(k) and the shiftsignal SH_(k) go to L level (VSS). As a result, the gate line GL_(k)returns to a non-selected state.

When the shift signal SH_(k) was activated, the next stage (unit shiftregister SR_(k+1)) is in a set state, and the shift signal SH_(k+1) ofthe next stage goes to H level when the second clock signal /CLK isactivated next.

Then, in this unit shift register SR_(k), the transistor Q4 turns on,and the nodes N1 and N1S are discharged to L level (VSS). In response,the transistor Q7 turns off, and the node N2 is charged by thetransistor Q6 to go to H level (VDD-Vth). As a result, the unit shiftregister SR_(k) returns to the reset state, and the transistors Q1 andQ1S are off and the transistors Q2 and Q2S are on.

After that, the unit shift register SR_(k) is kept in the reset stateuntil the shift signal SH_(k−1) of the previous stage is activated inthe next frame period. This is because the half latch circuit formed ofthe transistors Q5 to Q7 holds the nodes N1 and N1S at L level and thenode N2 at H level. Accordingly, in the non-selected period of the gateline GL_(k), the gate line driving signal G_(k) and the shift signalSH_(k) are both kept at L level with low impedance.

The unit shift register SR_(k) described above can be summarized asfollows. That is to say, the unit shift register SR_(k) goes in a setstate in response to the activation of the signal at the input terminalIN (input signal). In the set state, the transistors Q1 and Q1S are onand the transistors Q2 and Q2S are off. Accordingly, when the signal atthe first clock terminal CK (the first clock signal) is activated, thegate driving signal G_(k) goes to an active level, and when the signalat the second clock terminal CKS (the second clock signal) is activated,the shift signal SH_(k) goes to an active level. Then, when the signalat the reset terminal RST (reset signal) is activated, it returns to thereset state, and after that the gate line driving signal G_(k) and theshift signal SH_(k) are kept at L level.

In a display effective period, the first clock signals CLK and /CLK areactivated like the second clock signals CLKS and /CLKS, and so the unitshift register SR_(k) in a set state (the transistors Q1 and Q1S are onand the transistors Q2 and Q2S are off) simultaneously activates thegate line driving signal G_(k) and the shift signal SH_(k).

On the other hand, in a display ineffective period, only the secondclock signals CLKS and /CLKS are activated and the first clock signalsCLK and /CLK are kept at L level. Accordingly, even when the unit shiftregister SR_(k) enters a set state, the first output terminal OUT is notcharged and the gate line driving signal G_(k) does not change from Llevel. Accordingly, in a display ineffective period, the shift registerSR_(k) activates only the shift signal SH_(k).

In the gate line driving circuit 30 of FIG. 2, a shift signal SH of theprevious stage is inputted to the input terminal IN of each unit shiftregister SR, so that the shift operation of the shift signals SH isnormally performed even when a period when the gate line driving signalsG are not activated exists, and the shift signals are activated as SH₁,SH₂, SH₃, . . . Accordingly, when a partial display is made, the shiftsignals SH₁, SH₂, SH₃, . . . are all activated in this order, and thegate line driving signals G₁, G₂, G₃, . . . are activated only when theyare in a display effective period (in selected periods of pixel linesrequiring display).

FIG. 4 is a signal waveform diagram illustrating the operation of thegate line driving circuit 30 of this preferred embodiment, and it showsan example in which a partial display is made where only the kth to(k+3)th pixel lines display.

As described above, the first clock signals CLK and /CLK are activatedlike the second clock signals CLKS and /CLKS only in display effectiveperiods. For example, when the display is made only with the kth to(k+3)th pixel lines, as shown in FIG. 4, the first clock signals CLK and/CLK are activated in synchronization with the second clock signals CLKSand /CLKS from the beginning of the selected period of the kth line (theactive period of the shift signal SH_(k)) to the end of the selectedperiod of the (k+3)th line (the active period of the shift signalSH_(k+3)). Thus, the gate line driving signals G_(k) to G_(k+3) areactivated in the selected periods of the gate lines GL_(k) to G_(k+3).The gate line driving signals G of the other lines are kept at aninactive level throughout the frame period.

Though not graphically shown, when an entire screen display is made, thefirst clock signals CLK and /CLK are always activated like the secondclock signals CLKS and /CLKS. Accordingly, the gate line driving signalsG₁, G₂, G₃, . . . are all sequentially activated with the same timing asthe shift signals SH₁, SH₂, SH₃, . . .

Now, pay attention to the operation of the transistor Q8 in the unitshift register SR_(k) in a display ineffective period. In a displayineffective period, the first clock terminal CK and the first outputterminal OUT do not change from L level, and so the node N1 is notstepped up by the coupling through the gate capacitance of thetransistor Q1 and the capacitance element C. Accordingly, when thepotential of the node N1S is stepped up from VDD-Vth as the level of thesecond output terminal OUTS rises, the potential of the node N1 does notchange from VDD-Vth.

That is to say, when the node N1S is stepped up, the source (node N1)potential of the transistor Q8 is VDD-Vth and the gate potential is VDD,and the gate-source voltage is Vth. In this state, only slight current,or sub-threshold current, flows in the transistor Q8, and the transistorQ8 is substantially off. That is to say, almost no current flows fromthe node N1S to the node N1, and the node N1S and the node N1 aresubstantially separated. Accordingly, the parasitic capacitance of thenode N1S is small, and the node N1S is sufficiently stepped up as in adisplay effective period.

It is important that the high-side power-supply potential VDD1 suppliedto the gate of the transistor Q8 be set not more than the potential ofthe active level of the shift signal SH_(k−1) of the previous stageinputted to the input terminal IN (corresponding to the active level ofthe second clock signals CLKS, /CLKS). In this preferred embodiment, thehigh-side power-supply potential VDD 1 and the potential of the activelevel of the shift signal SH_(k−1) of the previous stage are assumed tobe equal, and its value is defined as VDD. When the gate potential ofthe transistor Q8 is larger than VDD, the transistor Q8 does not turnoff when the node N1S is stepped up in a display ineffective period, andthe node N1S and the node N1 are not separated, and the above-describedeffect is not obtained.

On the other hand, when the gate potential of the transistor Q8 is lowerthan VDD, the potential of the node N1S after charged by the transistorQ8 is lower accordingly, which is not preferred, either. Accordingly, itis preferred that the gate potential of the transistor Q8 be higher in arange not exceeding VDD.

As describe so far, in the unit shift register SR_(k) of this preferredembodiment, the transistor Q8 separates the nodes N1 and N1S when thenode N1S is stepped up in response to activation of the shift signalSH_(k). Accordingly, the node N1S is sufficiently stepped up and therising rate of the shift signal SH_(k) is sufficiently high. This effectis obtained both in display effective periods and display ineffectiveperiods. That is to say, even in a display ineffective period, therising rate of the shift signal SH is not lowered, and the reduction ofoperational margin of the gate line driving circuit 30 is prevented.

This preferred embodiment has shown an example in which the shiftregisters of the gate line driving circuit 30 are driven by usingtwo-phase clock signals, but three- or more-phase clock signals can beused, of course.

(First Modification)

In the unit shift register SR_(k) of FIG. 3, the drain of the transistorQ3 is connected to the second power-supply terminal S2 to which thegiven high-side power-supply potential VDD1 is supplied, but, as shownin FIG. 5, it may be connected to the input terminal IN to which theshift signal SH_(k−1) of the previous stage is supplied. Then, theinterconnection for supplying the high-side power-supply potential VDD1to the transistor Q3 can be omitted and the circuit layout becomeseasier.

In the configuration of FIG. 3, as compared with that of FIG. 5, theload capacitance that the shift signals SH of the unit shift registersSR drive is reduced, and the rising rate and falling rate of the shiftsignals SH of individual stages are enhanced.

(Second Modification)

In scanning line driving circuits of liquid-crystal display apparatusesand the like, transistors (a-Si transistors) formed of amorphous silicon(a-Si) are widely used. Such a-Si transistors have a property that thethreshold voltage shifts when the gate is biased in a direct currentmanner, and it might cause malfunction of the scanning line drivingcircuit.

In the circuit of FIG. 3, for example, the gate of the transistor Q8 isalways positively biased, so that the threshold voltage of thetransistor Q8 shifts in the positive direction. In this case, theon-state resistance of the transistor Q8 becomes higher and the rates ofcharging/discharging of the node N1S might be lowered.

FIG. 6 is a circuit diagram of a unit shift register SR_(k) according toa second modification of this preferred embodiment. In this unit shiftregister SR_(k), as compared with the circuit of FIG. 5, the gate of thetransistor Q8 is connected to the input terminal IN, and a transistorQ5S having its gate connected to the node N2 is connected between thenode N1S and the first power-supply terminal S1. This modification isapplicable also to the circuit of FIG. 3.

In the unit shift register SR of FIG. 6, the gate of the transistor Q8is positively biased only in periods in which the node N1 goes to Hlevel and the transistor Q8 charges the node N1S. That is to say, thegate of the transistor Q8 is not biased in a direct current manner, andthe shift of the threshold voltage of the transistor Q8 is prevented.

However, unlike the circuits of FIG. 3 and FIG. 5, the transistor Q8cannot discharge the node N1S, and so it requires the transistor Q5Sthat discharges the node N1S when the unit shift register SR is in areset state (when the node N2 is at H level). Accordingly, it should benoted that it requires one more transistor as compared with FIGS. 3 and5.

(Third Modification)

FIG. 7 is a circuit diagram of a unit shift register SR_(k) according toa third modification of the present invention. In place of thetransistor Q8 of FIG. 5, this unit shift register SR_(k) includestransistors Q3S and Q5S connected to the node N1S. The transistor Q3S isconnected between the node N1S and the input terminal IN, and its gateis connected to the input terminal IN. The transistor Q5S is connectedbetween the node N1S and the first power-supply terminal S1, and itsgate is connected to the node N2.

This modification is applicable also to the circuit of FIG. 3. The drainof the transistor Q3S may be connected to the second power-supplyterminal S2 or the third power-supply terminal S3.

In the shift register SR of FIG. 7, the transistor Q3S charges the nodeN1S in response to activation of the shift signal SH_(k−1) of theprevious stage, and the transistor Q5S discharges the node N1S when thenode N2 goes to H level. That is to say, the two transistors Q3S and Q5Soperate like the transistor Q8 in the circuits of FIG. 3 and FIG. 5.

According to this modification, the nodes N1 and N1S are completelyseparated, and so the gate capacitance of the transistor Q1 and thecapacitance element C do not influence the step-up of the node N1S.Accordingly, even in a display ineffective period, the rising rate ofthe shift signal SH_(k) is not lowered, and the reduction of operationalmargin of the gate line driving circuit 30 is prevented. However, itshould be noted that it requires one more transistor as compared withFIGS. 3 and 5.

(Fourth Modification)

In this modification, the present invention is applied to a shiftregister that can change the shift direction of signals. A gate linedriving circuit 30 formed of such a shift register is capable ofscanning in both directions. The operation of shifting signals in theprevious to following direction (the order of unit shift registers SR₁,SR₂, SR₃, . . . ) is defined as “forward shift”, and the operation ofshifting signals in the following to previous direction (the order ofunit shift registers SR_(n), SR_(n−1), SR_(n−2), . . . ) is defined as“backward shift”.

FIG. 8 is a circuit diagram of a unit shift register SR_(k) according tothis modification. The unit shift register SR_(k) is configuredsimilarly to that of FIG. 3, except for the following respects.

That is to say, the unit shift register SR_(k) has a first inputterminal IN 1 receiving the shift signal SH_(k−1) of the previous stage,a second input terminal IN2 receiving the shift signal SH_(k+1) of thenext stage, and first and second voltage signal terminals T1 and T2supplied respectively with given first and second voltage signals Vn andVr. The transistor Q3 is connected between the node N1 and the firstvoltage signal terminal T1, and its gate is connected to the first inputterminal IN1. The transistor Q4 is connected between the node N1 and thesecond voltage signal terminal T2, and its gate is connected to thesecond input terminal IN2.

The first and second voltage signals Vn and Vr are control signals fordetermining the direction of shift of signals (scanning direction). Whenthe unit shift register SR_(k) performs forward shift, the first voltagesignal Vn is set at H level and the second voltage signal Vr is set at Llevel. When it performs backward shift, the first voltage signal Vn isset at L level and the second voltage signal Vr is set at H level.

When the first voltage signal Vn is at H level and the second voltagesignal Vr is at L level, the shift register SR_(k) of FIG. 8 isequivalent to the circuit of FIG. 3, and forward shift is enabled. Onthe other hand, when the first voltage signal Vn is at L level and thesecond voltage signal Vr is at H level, the functions of the transistorsQ3 and Q4 in the unit shift register SR_(k) of FIG. 8 are opposite tothose in the circuit of FIG. 3. That is to say, the transistor Q4functions to charge the node N1 and the transistor Q3 functions todischarge the node N1. As a result, backward shift is enabled.

(Fifth Modification)

In the circuit of FIG. 3, the source of the transistor Q3, the drains ofthe transistors Q4 and Q5, and the gate of the transistor Q7 are allconnected to the node N1, but one or more of them may be connected tothe node N1S. FIG. 9 shows an example in which all of them are connectedto the node N1S.

When the source of the transistor Q3, the drains of the transistors Q4and Q5, and the gate of the transistor Q7 are connected to the node N1S,the parasitic capacitance of the node N1 is reduced accordingly, and thenode N1 is stepped up to higher potential, and the driving capability ofthe transistor Q1 is enhanced. Then, the rate of level transition of thegate line driving signal G is increased and the gate line GL can bedriven at a higher rate.

On the other hand, the parasitic capacitance of the node N1S isincreased, and so it should be noted that the rate of level transitionof the shift signal SH is lowered. Whether the source of the transistorQ3, the drains of the transistors Q4 and Q5, and the gate of thetransistor Q7 should be connected to the node N1 or N1S can bedetermined by considering characteristics of the display apparatus.

This modification is applicable also to the unit shift registers SR ofthe above-described modifications (FIGS. 5 to 8). However, the source ofthe transistor Q3 in FIG. 7 must be connected to the node N1. This isbecause, in the configuration of FIG. 7, the nodes N1 and N1S have to becharged with separate transistors.

Also, the drain of the transistor Q5 in FIG. 6 and FIG. 7 must beconnected to the node N1. This is because the configurations of FIGS. 6and 7 require separate transistors which bring the nodes N1 and N1S to Llevel with low impedance when the unit shift register SR is in a resetstate.

Also, in the circuits of FIGS. 6 and 7, the drain of the transistor Q4and the gate of the transistor Q7 must be connected to the same side.This is because, in the configurations of FIGS. 6 and 7, the nodes N1and N1S are separated when the transistor Q4 turns on, and so thetransistor Q7 cannot be turned off unless the drain of the transistor Q4and the gate of the transistor Q7 are connected.

(Sixth Modification)

In this modification, the third modification (FIG. 7) is applied to ashift register capable of changing the shift direction of signals(fourth modification (FIG. 8)).

FIG. 10 is a circuit diagram of a unit shift register SR_(k) accordingto a sixth modification of this preferred embodiment. Like that of FIG.8, this unit shift register SR_(k) has a first input terminal IN1receiving the shift signal SH_(k−1) of the previous stage, a secondinput terminal IN2 receiving the shift signal SH_(k+1) of the nextstage, and first and second voltage signal terminals T1 and T2 suppliedrespectively with first and second voltage signals Vn and Vr.

The circuit configuration of this unit shift register SR_(k) differsfrom that of FIG. 7 in the following respects. That is to say, thetransistor Q3 is connected between the node N1 and the first voltagesignal terminal T1, and its gate is connected to the first inputterminal IN1. The transistor Q4 is connected between the node N1 and thesecond voltage signal terminal T2, and its gate is connected to thesecond input terminal IN2. The transistor Q3S is connected between thenode N1S and the first voltage signal terminal T1, and its gate isconnected to the first input terminal IN1. Also, it includes atransistor Q4S having its gate connected to the second input terminalIN2 and connected between the node N1S and the second voltage signalterminal T2.

When the first voltage signal Vn is at H level and the second voltagesignal Vr is at L level, the unit shift register SR_(k) of FIG. 10 isequivalent to the circuit of FIG. 7, and forward shift is enabled.However, while the node N1S is discharged by the transistor Q5S in FIG.7, it is discharged mainly by the transistor Q4S in the circuit of FIG.10 (the transistor Q5S mainly functions to keep the node N1S at L levelwith low impedance in a reset state).

On the other hand, when the first voltage signal Vn is at L level andthe second voltage signal Vr is at H level, in the unit shift registerSR_(k) in FIG. 10, the functions of the transistors Q3, Q4, Q3S and Q4Sare opposite to those in the case of forward shift. That is to say, thetransistors Q4 and Q4S respectively function to charge the nodes N1 andN1S, and the transistors Q3 and Q3S respectively function to dischargethe nodes N1 and N1S. As a result, backward shift is enabled.

In the circuit of FIG. 10, the gate of the transistor Q7 may beconnected to the node N1S.

While the invention has been described in detail, the foregoingdescription is in all aspects illustrative and not restrictive. It isunderstood that numerous other modifications and variations can bedevised without departing from the scope of the invention.

What is claimed is:
 1. A shift register circuit comprising: first and second input terminals, first and second output terminals, and first and second clock terminals; first and second voltage signal terminals respectively supplied with first and second voltage signals that are complementary to each other; a first transistor that supplies said first output terminal with a first clock signal inputted to said first clock terminal; a second transistor that supplies said second output terminal with a second clock signal inputted to said second clock terminal; a third transistor that supplies said first voltage signal to a first node to which a control electrode of said first transistor connects or a second node to which a control electrode of said second transistor connects, in response to activation of a first input signal inputted to said first input terminal; a fourth transistor that supplies said second voltage signal to said first node or said second node in response to activation of a second input signal inputted to said second input terminal; and a fifth transistor connected between said first node and said second node, wherein said first clock signal and said second clock signal have a same phase, and a setting is possible such that only said second clock signal is activated and the first clock signal is kept at an inactive level in a particular period.
 2. The shift register circuit according to claim 1, wherein a control electrode of said fifth transistor is connected to a power supply of a given potential that is not more than an active level of said second clock signal.
 3. The shift register circuit according to claim 1, wherein, when said second output terminal is charged, said fifth transistor electrically separates said first node and second node.
 4. The shift register circuit according to claim 1, further comprising: a sixth transistor that discharges said first output terminal; and a seventh transistor that discharges said second output terminal; wherein a control electrode of said sixth transistor and a control electrode of said seventh transistor are connected to each other. 